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Versions: 00 01 02 03                                                   
INTERNET-DRAFT                                                 T. Maufer
Expire in six months                                          C. Semeria
Category: Informational                                 3Com Corporation
                                                              March 1997

               Introduction to IP Multicast Routing


Status of this Memo

This document is an Internet Draft.  Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas, and
its Working Groups.  Note that other groups may also distribute working
documents as Internet Drafts.

Internet Drafts are draft documents valid for a maximum of six months.
Internet Drafts may be updated, replaced, or obsoleted by other
documents at any time.  It is not appropriate to use Internet Drafts as
reference material or to cite them other than as a "working draft" or
"work in progress."

To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the internet-drafts Shadow
Directories on:

    ftp.is.co.za                (Africa)
    nic.nordu.net               (Europe)
    ds.internic.net      (US East Coast)
    ftp.isi.edu          (US West Coast)
    munnari.oz.au          (Pacific Rim)


This document is introductory in nature.  We have not attempted to
describe every detail of each protocol, rather to give a concise
overview in all cases, with enough specifics to allow a reader to grasp
the essential details and operation of protocols related to multicast
IP.  Every effort has been made to ensure the accurate representation of
any cited works, especially any works-in-progress.  For the complete
details, we refer you to the relevant specification(s).

If internet-drafts are cited in this document, it is only because they
are the only sources of certain technical information at the time of
this writing.  We expect that many of the internet-drafts which we have
cited will eventually become RFCs.  See the shadow directories above for
the status of any of these drafts, their follow-on drafts, or possibly
the resulting RFCs.

Maufer & Semeria                                                [Page 1]

INTERNET-DRAFT      Introduction to IP Multicast Routing      March 1997


The first part of this paper describes the benefits of multicasting,
the MBone, Class D addressing, and the operation of the Internet Group
Management Protocol (IGMP).  The second section explores a number of
different techniques that may potentially be employed by multicast
routing protocols:

    o  Flooding
    o  Spanning Trees
    o  Reverse Path Broadcasting (RPB)
    o  Truncated Reverse Path Broadcasting (TRPB)
    o  Reverse Path Multicasting (RPM)
    o  "Shared-Tree" Techniques

The third part contains the main body of the paper.  It describes how
the previous techniques are implemented in multicast routing protocols
available today (or under development).

    o  Distance Vector Multicast Routing Protocol (DVMRP)
    o  Multicast Extensions to OSPF (MOSPF)
    o  Protocol-Independent Multicast - Dense Mode (PIM-DM)
    o  Protocol-Independent Multicast - Sparse Mode (PIM-SM)
    o  Core-Based Trees (CBT)

                          Table of Contents

1  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  INTRODUCTION
1.1  . . . . . . . . . . . . . . . . . . . . . . . . .  Multicast Groups
1.2  . . . . . . . . . . . . . . . . . . . . . Group Membership Protocol
1.3  . . . . . . . . . . . . . . . . . . . . Multicast Routing Protocols
1.3.1  . . . . . . . . . . .  Multicast Routing vs. Multicast Forwarding
2.1  . . . . . . . . . . . . . . . . . . . . . . . Reducing Network Load
2.2  . . . . . . . . . . . . . . . . . . . . . . . .  Resource Discovery
2.3  . . . . . . . . . . . . . . .  Support for Datacasting Applications
3  . . . . . . . . . . . . . . THE INTERNET'S MULTICAST BACKBONE (MBone)
4  . . . . . . . . . . . . . . . . . . . . .  . . . MULTICAST ADDRESSING
4.1  . . . . . . . . . . . . . . . . . . . . . .  . .  Class D Addresses
4.2  . . . . . . .  Mapping a Class D Address to an IEEE-802 MAC Address
4.3  . . . . . . . . .  Transmission and Delivery of Multicast Datagrams
5  . . . . . . . . . . . . . . INTERNET GROUP MANAGEMENT PROTOCOL (IGMP)
5.1  . . . . . . . . . . . . . . . . . . . . . . . . . .  IGMP Version 1
5.2  . . . . . . . . . . . . . . . . . . . . . . . . . .  IGMP Version 2
5.3  . . . . . . . . . . . . . . . . . . . . . . . . . .  IGMP Version 3
6  . . . . . . . . . . . . . . . . . . . MULTICAST FORWARDING TECHNIQUES
6.1  . . . . . . . . . . . . . . . . . . . . . "Simpleminded" Techniques
6.1.1  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Flooding
6.1.2  . . . . . . . . . . . . . . . . . . . . . . . . . . Spanning Tree
6.2  . . . . . . . . . . . . . . . . . . .  Source-Based Tree Techniques

Maufer & Semeria                                                [Page 2]

INTERNET-DRAFT      Introduction to IP Multicast Routing      March 1997

6.2.1  . . . . . . . . . . . . . . . . . Reverse Path Broadcasting (RPB)  . . . . . . . . . . . . .  Reverse Path Broadcasting: Operation  . . . . . . . . . . . . . . . . . RPB: Benefits and Limitations
6.2.2  . . . . . . . . . . .  Truncated Reverse Path Broadcasting (TRPB)
6.2.3  . . . . . . . . . . . . . . . . . Reverse Path Multicasting (RPM)  . . . . . . . . . . . . . . . . . . . . . . . . . . . Operation  . . . . . . . . . . . . . . . . . . . . . . . . . . Limitations
6.3  . . . . . . . . . . . . . . . . . . . . . .  Shared Tree Techniques
6.3.1  . . . . . . . . . . . . . . . . . . . . . . . . . . . . Operation
6.3.2  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Benefits
6.3.3  . . . . . . . . . . . . . . . . . . . . . . . . . . . Limitations
7  . . . . . . . . . . . . . . . . . . .  "DENSE MODE" ROUTING PROTOCOLS
7.1  . . . . . . . .  Distance Vector Multicast Routing Protocol (DVMRP)
7.1.1  . . . . . . . . . . . . . . . . .  Physical and Tunnel Interfaces
7.1.2  . . . . . . . . . . . . . . . . . . . . . . . . . Basic Operation
7.1.3  . . . . . . . . . . . . . . . . . . . . .  DVMRP Router Functions
7.1.4  . . . . . . . . . . . . . . . . . . . . . . . DVMRP Routing Table
7.1.5  . . . . . . . . . . . . . . . . . . . . .  DVMRP Forwarding Table
7.2  . . . . . . . . . . . . . . .  Multicast Extensions to OSPF (MOSPF)
7.2.1  . . . . . . . . . . . . . . . . . . Intra-Area Routing with MOSPF  . . . . . . . . . . . . . . . . . . . . .  Local Group Database  . . . . . . . . . . . . . . . . . Datagram's Shortest Path Tree  . . . . . . . . . . . . . . . . . . . . . . .  Forwarding Cache
7.2.2  . . . . . . . . . . . . . . . . . . Mixing MOSPF and OSPF Routers
7.2.3  . . . . . . . . . . . . . . . . . . Inter-Area Routing with MOSPF  . . . . . . . . . . . . . . . . Inter-Area Multicast Forwarders  . . . . . . . . . . .  Inter-Area Datagram's Shortest Path Tree
7.2.4  . . . . . . . . . Inter-Autonomous System Multicasting with MOSPF
7.3  . . . . . . . . . . . . . . .  Protocol-Independent Multicast (PIM)
7.3.1  . . . . . . . . . . . . . . . . . . . . PIM - Dense Mode (PIM-DM)
8  . . . . . . . . . . . . . . . . . . . "SPARSE MODE" ROUTING PROTOCOLS
8.1  . . . . . . . Protocol-Independent Multicast - Sparse Mode (PIM-SM)
8.1.1  . . . . . . . . . . . . . .  Directly Attached Host Joins a Group
8.1.2  . . . . . . . . . . . . Directly Attached Source Sends to a Group
8.1.3  . . . . . . .  Shared Tree (RP-Tree) or Shortest Path Tree (SPT)?
8.1.4  . . . . . . . . . . . . . . . . . . . . . . .   Unresolved Issues
8.2  . . . . . . . . . . . . . . . . . . . . . .  Core Based Trees (CBT)
8.2.1  . . . . . . . . . . . . . . . . . . Joining a Group's Shared Tree
8.2.2  . . . . . . . . . . . . . . . . . . . . .  Data Packet Forwarding
8.2.3  . . . . . . . . . . . . . . . . . . . . . . .  Non-Member Sending
8.2.4  . . . . . . . . . . . . . . . . .  CBT Multicast Interoperability
9.1  . . . . . . . . . . . . . Requirements for Multicast Border Routers
10  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . REFERENCES
10.1  . . . . . . . . . . . . . . . . . . . Requests for Comments (RFCs)
10.2  . . . . . . . . . . . . . . . . . . . . . . . . .  Internet-Drafts
10.3  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Textbooks
10.4  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Other
11  . . . . . . . . . . . . . . . . . . . . . .  SECURITY CONSIDERATIONS
12  . . . . . . . . . . . . . . . . . . . . . . . . . . ACKNOWLEDGEMENTS
13  . . . . . . . . . . . . . . . . . . . . . . . . . AUTHORS' ADDRESSES

Maufer & Semeria                                                [Page 3]

INTERNET-DRAFT      Introduction to IP Multicast Routing      March 1997


There are three fundamental types of IPv4 addresses:  unicast,
broadcast, and multicast.  A unicast address is used to transmit a
packet to a single destination.  A broadcast address is used to send a
datagram to an entire subnetwork.  A multicast address is designed to
enable the delivery of datagrams to a set of hosts that have been
configured as members of a multicast group across various

Multicasting is not connection-oriented.  A multicast datagram is
delivered to destination group members with the same "best-effort"
reliability as a standard unicast IP datagram.  This means that
multicast datagrams are not guaranteed to reach all members of a group,
nor to arrive in the same order in which they were transmitted.

The only difference between a multicast IP packet and a unicast IP
packet is the presence of a 'group address' in the Destination Address
field of the IP header.  Instead of a Class A, B, or C IP destination
address, multicasting employs a Class D address format, which ranges
from to

1.1 Multicast Groups

Individual hosts are free to join or leave a multicast group at any
time.  There are no restrictions on the physical location or the number
of members in a multicast group.  A host may be a member of more than
one multicast group at any given time and does not have to belong to a
group to send packets to members of a group.

1.2 Group Membership Protocol

A group membership protocol is employed by routers to learn about the
presence of group members on their directly attached subnetworks.  When
a host joins a multicast group, it transmits a group membership protocol
message for the group(s) that it wishes to receive, and sets its IP
process and network interface card to receive frames addressed to the
multicast group.  This receiver-initiated join process has excellent
scaling properties since, as the multicast group increases in size, it
becomes ever more likely that a new group member will be able to locate
a nearby branch of the multicast delivery tree.

[This space was intentionally left blank.]

Maufer & Semeria                                                [Page 4]

INTERNET-DRAFT      Introduction to IP Multicast Routing      March 1997

                            _    _    _    _
                           |_|  |_|  |_|  |_|
                           '-'  '-'  '-'  '-'
                            |    |    |    |
                          <- - - - - - - - - ->
                                /     \
         _  ^                 +         +              ^  _
        |_|-|               /            \             |-|_|
        '_' |             +                +           | '_'
         _  |          v                     v         |  _
        |_|-|- - >|Router| <- + - + - + -> |Router|<- -|-|_|
        '_' |                                          | '_'
         _  |                                          |  _
        |_|-|                                          |-|_|
        '_' |                                          | '_'
            v                                          v


<- - - -> Group Membership Protocol
<-+-+-+-> Multicast Routing Protocol

Figure 1: Multicast IP Delivery Service

1.3 Multicast Routing Protocols

Multicast routers execute a multicast routing protocol to define
delivery paths that enable the forwarding of multicast datagrams
across an internetwork.

1.3.1  Multicast Routing vs. Multicast Forwarding

Multicast routing protocols establish or help establish the distribution
tree for a given group, which enables multicast forwarding of packets
addressed to the group.  In the case of unicast, routing protocols are
also used to build a forwarding table (commonly called a routing table).
Unicast destinations are entered in the routing table, and associated
with a metric and a next-hop router toward the destination.  The key
difference between unicast forwarding and multicast forwarding is that
multicast packets must be forwarded away from their source.  If a packet
is ever forwarded back toward its source, a forwarding loop could have
formed, possibly leading to a multicast "storm."

Maufer & Semeria                                                [Page 5]

INTERNET-DRAFT      Introduction to IP Multicast Routing      March 1997

Each routing protocol constructs a forwarding table in its own way; the
forwarding table tells each router that for a certain source, or for a
given source sending to a certain group (called a (source, group) pair),
packets are expected to arrive on a certain "inbound" or "upstream"
interface and must be copied to certain (set of) "outbound" or
"downstream" interface(s) in order to reach all known subnetworks with
group members.


Today, the majority of Internet applications rely on point-to-point
transmission.  The utilization of point-to-multipoint transmission has
traditionally been limited to local area network applications.  Over the
past few years the Internet has seen a rise in the number of new
applications that rely on multicast transmission.  Multicast IP
conserves bandwidth by forcing the network to do packet replication only
when necessary, and offers an attractive alternative to unicast
transmission for the delivery of network ticker tapes, live stock
quotes, multiparty videoconferencing, and shared whiteboard applications
(among others). It is important to note that the applications for IP
Multicast are not solely limited to the Internet.  Multicast IP can also
play an important role in large commercial internetworks.

2.1 Reducing Network Load

Assume that a stock ticker application is required to transmit packets
to 100 stations within an organization's network.  Unicast transmission
to this set of stations will require the periodic transmission of 100
packets where many packets may in fact be traversing the same link(s).
Multicast transmission is the ideal solution for this type of
application since it requires only a single packet stream to be
transmitted by the source which is replicated at forks in the multicast
delivery tree.

Broadcast transmission is not an effective solution for this type of
application since it affects the CPU performance of each and every
station that sees the packet.  Besides, it wastes bandwidth.

2.2 Resource Discovery

Some applications utilize multicast instead of broadcast transmission
to transmit packets to group members residing on the same subnetwork.
However, there is no reason to limit the extent of a multicast
transmission to a single LAN.  The time-to-live (TTL) field in the IP
header can be used to limit the range (or "scope") of a multicast

Maufer & Semeria                                                [Page 6]

INTERNET-DRAFT      Introduction to IP Multicast Routing      March 1997

2.3 Support for Datacasting Applications

Since 1992, the IETF has conducted a series of "audiocast" experiments
in which live audio and video were multicast from the IETF meeting site
to destinations around the world.  In this case, "datacasting" takes
compressed audio and video signals from the source station and transmits
them as a sequence of UDP packets to a group address.  Multicast
delivery today is not limited to audio and video.  Stock quote systems
are one example of a (connectionless) data-oriented multicast
application.  Someday reliable multicast transport protocols may
facilitate efficient inter-computer communication.  Reliable multicast
transport protocols are currently an active area of research and


The Internet Multicast Backbone (MBone) is an interconnected set of
subnetworks and routers that support the delivery of IP multicast
traffic.  The goal of the MBone is to construct a semipermanent IP
multicast testbed to enable the deployment of multicast applications
without waiting for the ubiquitous deployment of multicast-capable
routers in the Internet.

The MBone has grown from 40 subnets in four different countries in 1992,
to more than 3400 subnets in over 25 countries by   March 1997.  With
new multicast applications and multicast-based services appearing, it
seems likely that the use of multicast technology in the Internet will
keep growing at an ever-increasing rate.

The MBone is a virtual network that is layered on top of sections of the
physical Internet.  It is composed of islands of multicast routing
capability connected to other islands by virtual point-to-point links
called "tunnels."  The tunnels allow multicast traffic to pass through
the non-multicast-capable parts of the Internet.  Tunneled IP multicast
packets are encapsulated as IP-over-IP (i.e., the protocol number is set
to 4) so they look like normal unicast packets to intervening routers.
The encapsulation is added on entry to a tunnel and stripped off on exit
from a tunnel.  This set of multicast routers, their directly-connected
subnetworks, and the interconnecting tunnels comprise the MBone.

Since the MBone and the Internet have different topologies, multicast
routers execute a separate routing protocol to decide how to forward
multicast packets.  The majority of the MBone routers currently use the
Distance Vector Multicast Routing Protocol (DVMRP), although some
portions of the MBone execute either Multicast OSPF (MOSPF) or the
Protocol-Independent Multicast (PIM) routing protocols.  The operation
of each of these protocols is discussed later in this paper.

Maufer & Semeria                                                [Page 7]

INTERNET-DRAFT      Introduction to IP Multicast Routing      March 1997


                           / |Island | \
                         /T/ |   A   | \T\
                        /U/   +++++++    \U\
                      /N/        |         \N\
                    /N/          |           \N\
                  /E/            |             \E\
                /L/              |               \L\
         ++++++++             +++++++            ++++++++
        | Island |           | Island| ---------| Island |
        |    B   |           |   C   |   Tunnel |   D    |
         ++++++++             +++++++  --------- ++++++++
               \ \               |
                 \T\             |
                   \U\           |
                    \N\          |
                      \N\     +++++++
                        \E\  |Island |
                          \L\|   E   |
                            \ +++++++

Figure 2: Internet Multicast Backbone (MBone)


As multicast routing software features become more widely available on
the routers of the Internet, providers may gradually decide to use
"native" multicast as an alternative to using lots of tunnels.

The MBone carries audio and video multicasts of Internet Engineering
Task Force (IETF) meetings, NASA Space Shuttle Missions, US House and
Senate sessions, and live satellite weather photos.  There are public
and private sessions on the MBone.  Sessions that are menat for public
viewing or participation are announced via the session directory (SDR)
tool.  A user of this tool can see a list of current and future public
sessions provided the user is within the administrative scope of the


A multicast address is assigned to a set of receivers defining a
multicast group.  Senders use the multicast address as the destination
IP address of a packet that is to be transmitted to all group members.

Maufer & Semeria                                                [Page 8]

INTERNET-DRAFT      Introduction to IP Multicast Routing      March 1997

4.1 Class D Addresses

An IP multicast group is identified by a Class D address.  Class D
addresses have their high-order four bits set to "1110" followed by
a 28-bit multicast group ID.  Expressed in standard "dotted-decimal"
notation, multicast group addresses range from to (shorthand:

Figure 3 shows the format of a 32-bit Class D address.


      0 1 2 3                                                      31
     |1|1|1|0|                   Multicast Group ID                  |
             |------------------------28 bits------------------------|

Figure 3: Class D Multicast Address Format

The Internet Assigned Numbers Authority (IANA) maintains a list of
registered IP multicast groups.  The base address is reserved
and cannot be assigned to any group.  The block of multicast addresses
ranging from to is reserved for permanent
assignment to various uses, including routing protocols and other
protocols that require a well-known permanent address.  Multicast
routers should not forward any multicast datagram with destination
addresses in this range, (regardless of the packet's TTL).

Some of the well-known groups include:

    "all systems on this subnet"
    "all routers on this subnet"
    "all DVMRP routers"      
    "all OSPF routers"       
    "all OSPF designated routers"
    "all RIP2 routers"       
    "all PIM routers"        
    "all CBT routers"        

The remaining groups ranging from to are
assigned to various multicast applications or remain unassigned.  From
this range, the addresses from to are being
reserved for various "administratively scoped" applications, not
necessarily Internet-wide applications.

Maufer & Semeria                                                [Page 9]

INTERNET-DRAFT      Introduction to IP Multicast Routing      March 1997

The complete list may be found in the Assigned Numbers RFC (RFC 1700 or
its successor) or at the IANA Web Site:


4.2 Mapping a Class D Address to an IEEE-802 MAC Address

The IANA has been allocated a reserved portion of the IEEE-802 MAC-layer
multicast address space.  All of the addresses in IANA's reserved block
begin with 01-00-5E (hex); to be clear, the range from 01-00-5E-00-00-00
to 01-00-5E-FF-FF-FF is reserved for IP multicast groups.

A simple procedure was developed to map Class D addresses to this
reserved MAC-layer multicast address block.  This allows IP multicasting
to easily take advantage of the hardware-level multicasting supported by
network interface cards.

The mapping between a Class D IP address and an IEEE-802 (e.g., FDDI,
Ethernet) MAC-layer multicast address is obtained by placing the low-
order 23 bits of the Class D address into the low-order 23 bits of
IANA's reserved MAC-layer multicast address block.  This simple
procedure removes the need for an explicit protocol for multicast
address resolution on LANs akin to ARP for unicast.  All LAN stations
know this simple transformation, and can easily send any IP multicast
over any IEEE-802-based LAN.

Figure 4 illustrates how the multicast group address
(or EA-8A-08-05 expressed in hex) is mapped into an IEEE-802 multicast
address.  Note that the high-order nine bits of the IP address are not
mapped into the MAC-layer multicast address.

The mapping in Figure 4 places the low-order 23 bits of the IP multicast
group ID into the low order 23 bits of the IEEE-802 multicast address.
Note that the mapping may place up to multiple IP groups into the same
IEEE-802 address because the upper five bits of the IP class D address
are not used.  Thus, there is a 32-to-1 ratio of IP class D addresses to
valid MAC-layer multicast addresses.  In practice, there is a small
chance of collisions, should multiple groups happen to pick class D
addresses that map to the same MAC-layer multicast address.  However,
chances are that higher-layer protocols will let hosts interpret which
packets are for them (i.e., the chances of two different groups picking
the same class D address and the same set of UDP ports is extremely
unlikely).  For example, the class D addresses (E0-0A-08-05)
and (E1-8A-08-05) map to the same IEEE-802 MAC-layer
multicast address (01-00-5E-0A-08-05) used in this example.

Maufer & Semeria                 Informational                 [Page 10]

INTERNET-DRAFT      Introduction to IP Multicast Routing      March 1997


   Class D Address: (EA-8A-08-05)

                                |    E      A   |   8
                  Class-D IP    |_______ _______|__ _ _ _
                     Address    |-+-+-+-+-+-+-+-|-+ - - -
                                |1 1 1 0 1 0 1 0|1
                                |-+-+-+-+-+-+-+-|-+ - - -
IEEE-802                           ....not.........
MAC-Layer                            ..............
Multicast                              ....mapped..
Address                                 ...........
|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+ - - -
|0 0 0 0 0 0 0 1|0 0 0 0 0 0 0 0|0 1 0 1 1 1 1 0|0
|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+ - - -
|_______ _______|_______ _______|_______ _______|_______
|   0       1   |   0       0   |   5       E   |   0

    [Address mapping below continued from half above]

         |   8       A   |   0       8   |   0      5    |
         |_______ _______|_______ _______|_______ _______|    Class-D IP
  - - - -|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|    Address
         |  0 0 0 1 0 1 0|0 0 0 0 1 0 0 0|0 0 0 0 0 1 0 1|
  - - - -|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|
            \____________           ____________________/
                         \___   ___/
                             \ /
                   23 low-order bits mapped

  - - - -|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|    IEEE-802
         |  0 0 0 1 0 1 0|0 0 0 0 1 0 0 0|0 0 0 0 0 1 0 1|    MAC-Layer
  - - - -|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|    Multicast
         |_______ _______|_______ _______|_______ _______|    Address
         |   0       A   |   0       8   |   0       5   |

Figure 4: Mapping between Class D and IEEE-802 Multicast Addresses

Maufer & Semeria                 Informational                 [Page 11]

INTERNET-DRAFT      Introduction to IP Multicast Routing      March 1997

4.3 Transmission and Delivery of Multicast Datagrams

When the sender and receivers are members of the same (LAN) subnetwork,
the transmission and reception of multicast frames is a straightforward
process.  The source station simply addresses the IP packet to the
multicast group, the network interface card maps the Class D address to
the corresponding IEEE-802 multicast address, and the frame is sent.
Receivers that wish to capture the frame notify their MAC and IP layers
that they want to receive datagrams addressed to the group.

Things become somewhat more complex when the sender is attached to one
subnetwork and receivers reside on different subnetworks. In this case,
the routers must implement a multicast routing protocol that permits the
construction of multicast delivery trees and supports multicast packet
forwarding.  In addition, each router needs to implement a group
membership protocol that allows it to learn about the existence of group
members on its directly attached subnetworks.


The Internet Group Management Protocol (IGMP) runs between hosts and
their immediately-neighboring multicast routers.  The mechanisms of the
protocol allow a host to inform its local router that it wishes to
receive transmissions addressed to a specific multicast group.  Also,
routers periodically query the LAN to determine if any group members are
still active.  If there is more than one IP multicast router on the LAN,
one of the routers is elected "querier" and assumes the responsibility of
querying the LAN for the presence of any group members.

Based on the group membership information learned from the IGMP, a
router is able to determine which (if any) multicast traffic needs to be
forwarded to each of its "leaf" subnetworks.  Multicast routers use this
information, in conjunction with a multicast routing protocol, to
support IP multicasting across the Internet.

5.1 IGMP Version 1

IGMP Version 1 was specified in RFC-1112.  According to the
specification, multicast routers periodically transmit Host Membership
Query messages to determine which host groups have members on their
directly-attached networks.  IGMP Query messages are addressed to the
all-hosts group ( and have an IP TTL = 1.  This means that
Query messages sourced from a router are transmitted onto the
directly-attached subnetwork but are not forwarded by any other
multicast routers.

When a host receives an IGMP Query message, it responds with a Host
Membership Report for each group to which it belongs, sent to each group
to which it belongs.  (This is an important point:  While IGMP Queries

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     Group 1                                   _____________________
       ____            ____                   |  multicast          |
      |    |          |    |                  |            router   |
      |_H2_|          |_H4_|                  |_____________________|
       ----            ----                      +-----+  |
         |               |                 <-----|Query|  |
         |               |                       +-----+  |
         |               |                                |
    |            |                |
    |            |                |
  ____         ____              ____
 |    |       |    |            |    |
 |_H1_|       |_H3_|            |_H5_|
  ----         ----              ----
 Group 2      Group 1           Group 1
              Group 2

Figure 5: Internet Group Management Protocol-Query Message

are sent to the "all hosts on this subnet" class D address (,
IGMP Reports are sent to the group(s) to which the host(s) belong.
IGMP Reports, like Queries, are sent with the IP TTL = 1, and thus are
not forwarded beyond the local subnetwork.)

In order to avoid a flurry of Reports, each host starts a randomly-
chosen Report delay timer for each of its group memberships.  If, during
the delay period, another Report is heard for the same group, every
other host in that group must reset its timer to a new random value.
This procedure spreads Reports out over a period of time and thus
minimizes Report traffic for each group that has at least one member on
a given subnetwork.

It should be noted that multicast routers do not need to be directly
addressed since their interfaces are required to promiscuously receive
all multicast IP traffic.  Also, a router does not need to maintain a
detailed list of which hosts belong to each multicast group; the router
only needs to know that at least one group member is present on a given
network interface.

Multicast routers periodically transmit IGMP Queries to update their
knowledge of the group members present on each network interface.  If
the router does not receive a Report from any members of a particular
group after a number of Queries, the router assumes that group members

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are no longer present on an interface.  Assuming this is a leaf subnet
(i.e., a subnet with group members but no multicast routers connecting
to additional group members further downstream), this interface is
removed from the delivery tree(s) for this group.  Multicasts will
continue to be sent on this interface only if the router can tell (via
multicast routing protocols) that there are additional group members
further downstream reachable via this interface.

When a host first joins a group, it immediately transmits an IGMP Report
for the group rather than waiting for a router's IGMP Query.  This
reduces the "join latency" for the first host to join a given group on
a particular subnetwork.  "Join latency" is measured from the time when
a host's first IGMP Report is sent, until the transmission of the first
packet for that group onto that host's subnetwork.  Of course, if the
group is already active, the join latency is precisely zero.

5.2 IGMP Version 2

IGMP version 2 was distributed as part of the Distance Vector Multicast
Routing Protocol (DVMRP) implementation ("mrouted") source code, from
version 3.3 through 3.8.  Initially, there was no detailed specification
for IGMP version 2 other than this source code.  However, the complete
specification has recently been published in <draft-ietf-idmr-igmp-
v2-06.txt> which will update the specification contained in the first
appendix of RFC-1112.  IGMP version 2 extends IGMP version 1 while
maintaining backward compatibility with version 1 hosts.

IGMP version 2 defines a procedure for the election of the multicast
querier for each LAN.  In IGMP version 2, the multicast router with the
lowest IP address on the LAN is elected the multicast querier.  In IGMP
version 1, the querier election was determined by the multicast routing

IGMP version 2 defines a new type of Query message:  the Group-Specific
Query.  Group-Specific Query messages allow a router to transmit a Query
to a specific multicast group rather than all groups residing on a
directly attached subnetwork.

Finally, IGMP version 2 defines a Leave Group message to lower IGMP's
"leave latency."  When the last host to respond to a Query with a Report
wishes to leave that specific group, the host transmits a Leave Group
message to the all-routers group ( with the group field set to
the group being left.  In response to a Leave Group message, the router
begins the transmission of Group-Specific Query messages on the
interface that received the Leave Group message.  If there are no
Reports in response to the Group-Specific Query messages, then (if this
is a leaf subnet) this interface is removed from the delivery tree(s)
for this group (as was the case of IGMP version 1).  Again, multicasts
will continue to be sent on this interface if the router can tell (via
multicast routing protocols) that there are additional group members
further downstream reachable via this interface.

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"Leave latency" is measured from a router's perspective.  In version 1
of IGMP, leave latency was the time from a router's hearing the last
Report for a given group, until the router aged out that interface from
the delivery tree for that group (assuming this is a leaf subnet, of
course).  Note that the only way for the router to tell that this was
the LAST group member is that no reports are heard in some multiple of
the Query Interval (this is on the order of minutes).  IGMP version 2,
with the addition of the Leave Group message, allows a group member to
more quickly inform the router that it is done receiving traffic for a
group.  The router then must determine if this host was the last member
of this group on this subnetwork.  To do this, the router quickly
queries the subnetwork for other group members via the Group-Specific
Query message.  If no members send reports after several of these Group-
Specific Queries, the router can infer that the last member of that
group has, indeed, left the subnetwork.  The benefit of lowering the
leave latency is that prune messages can be sent as soon as possible
after the last member host drops out of the group, instead of having to
wait for several minutes worth of Query intervals to pass.  If a group
was experiencing high traffic levels, it can be very beneficial to stop
transmitting data for this group as soon as possible.

5.3 IGMP Version 3

IGMP version 3 is a preliminary draft specification published in
<draft-cain-igmp-00.txt>.  IGMP version 3 introduces support for Group-
Source Report messages so that a host can elect to receive traffic from
specific sources of a multicast group.  An Inclusion Group-Source Report
message allows a host to specify the IP addresses of the specific
sources it wants to receive.  An Exclusion Group-Source Report message
allows a host to explicitly identify the sources that it does not want
to receive.  With IGMP version 1 and version 2, if a host wants to
receive any traffic for a group, the traffic from all sources for the
group must be forwarded onto the host's subnetwork.

IGMP version 3 will help conserve bandwidth by allowing a host to select
the specific sources from which it wants to receive traffic.  Also,
multicast routing protocols will be able to make use this information to
conserve bandwidth when constructing the branches of their multicast
delivery trees.

Finally, support for Leave Group messages first introduced in IGMP
version 2 has been enhanced to support Group-Source Leave messages.
This feature allows a host to leave an entire group or to specify the
specific IP address(es) of the (source, group) pair(s) that it wishes
to leave.  Note that at this time, not all existing multicast routing
protocols have mechanisms to support such requests from group members.
This is one issue that will be addressed during the development of
IGMP version 3.

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IGMP provides the final step in a multicast packet delivery service
since it is only concerned with the forwarding of multicast traffic from
a router to group members on its directly-attached subnetworks.  IGMP is
not concerned with the delivery of multicast packets between neighboring
routers or across an internetwork.

To provide an internetwork delivery service, it is necessary to define
multicast routing protocols.  A multicast routing protocol is
responsible for the construction of multicast delivery trees and
enabling multicast packet forwarding.  This section explores a number of
different techniques that may potentially be employed by multicast
routing protocols:

    o "Simpleminded" Techniques
       - Flooding
       - Spanning Trees

    o  Source-Based Tree (SBT) Techniques
       - Reverse Path Broadcasting (RPB)
       - Truncated Reverse Path Broadcasting (TRPB)
       - Reverse Path Multicasting (RPM)

    o "Shared-Tree" Techniques

Later sections will describe how these algorithms are implemented in the
most prevalent multicast routing protocols in the Internet today  (e.g.,
Distance Vector Multicast Routing Protocol (DVMRP), Multicast extensions
to OSPF (MOSPF), Protocol-Independent Multicast (PIM), and Core-Based
Trees (CBT).

6.1 "Simpleminded" Techniques

Flooding and Spanning Trees are two algorithms that can be used to build
primitive multicast routing protocols.  The techniques are primitive due
to the fact that they tend to waste bandwidth or require a large amount
of computational resources within the multicast routers involved.  Also,
protocols built on these techniques may work for small networks with few
senders, groups, and routers, but do not scale well to larger numbers of
senders, groups, or routers.  Also, the ability to handle arbitrary
topologies may not be present or may only be present in limited ways.

6.1.1 Flooding

The simplest technique for delivering multicast datagrams to all routers
in an internetwork is to implement a flooding algorithm. The flooding
procedure begins when a router receives a packet that is addressed to a
multicast group.  The router employs a protocol mechanism to determine
whether or not it has seen this particular packet before.  If it is the
first reception of the packet, the packet is forwarded on all interfaces

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(except the one on which it arrived) guaranteeing that the multicast
packet reaches all routers in the internetwork.  If the router has seen
the packet before, then the packet is discarded.

A flooding algorithm is very simple to implement since a router does not
have to maintain a routing table and only needs to keep track of the
most recently seen packets.  However, flooding does not scale for
Internet-wide applications since it generates a large number of
duplicate packets and uses all available paths across the internetwork
instead of just a limited number.  Also, the flooding algorithm makes
inefficient use of router memory resources since each router is required
to maintain a distinct table entry for each recently seen packet.

6.1.2 Spanning Tree

A more effective solution than flooding would be to select a subset of
the internetwork topology which forms a spanning tree.  The spanning
tree defines a structure in which only one active path connects any two
routers of the internetwork.  Figure 6 shows an internetwork and a
spanning tree rooted at router RR.

Once the spanning tree has been built, a multicast router simply
forwards each multicast packet to all interfaces that are part of the
spanning tree except the one on which the packet originally arrived.
Forwarding along the branches of a spanning tree guarantees that the
multicast packet will not loop and that it will eventually reach all
routers in the internetwork.

A spanning tree solution is powerful and would be relatively easy to
implement since there is a great deal of experience with spanning tree
protocols in the Internet community.  However, a spanning tree solution
can centralize traffic on a small number of links, and may not provide
the most efficient path between the source subnetwork and group members.
Also, it is computationally difficult to compute a spanning tree in
large, complex topologies.

6.2 Source-Based Tree Techniques

The following techniques all generate a source-based tree by various
means.  The techniques differ in the efficiency of the tree building
process, and the bandwidth and router resources (i.e., state tables)
used to build a source-based tree.

6.2.1 Reverse Path Broadcasting (RPB)

A more efficient solution than building a single spanning tree for the
entire internetwork would be to build a spanning tree for each potential
source [subnetwork].  These spanning trees would result in source-based
delivery trees emanating from the subnetworks directly connected to the

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A Sample Internetwork

                   / |\              / \
                  |  | \           /    \
                  |  |   \       /       \
                  |  |    \    /          \
                  |  |      \ /            \
                  |  |       #------#       \
                  |  |      /       | \      \
                  |  |     /        |  \      \
                  |   \   /         |   \-------#
                  |    \ /          |     -----/|
                  |     #-----------#----/      |
                  |    /|\---    --/|    \      |
                  |   / |    \  /    \    \     |
                  |  /   \    /\     |     \   /
                  | /      \ /   \   |      \ /
                  #---------#--   \  |   ----#
                               \   \ |  /
                                \--- #-/

A Spanning Tree for this Sample Internetwork

                     #                #
                      \              /
                       \           /
                         \       /
                          \    /
                            \ /
                                    | \
                                    |  \
                                    |   \-------#
                       /|           |    \
                      / |            \    \
                     /   \           |     \
                    /      \         |      \
                   #        #        |       #

#   Router
RR  Root Router

Figure 6: Spanning Tree

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source stations.  Since there are many potential sources for a group, a
different delivery tree is constructed rooted at each active source. Reverse Path Broadcasting: Operation

The fundamental algorithm to construct these source-based trees is
referred to as Reverse Path Broadcasting (RPB).  The RPB algorithm is
actually quite simple.  For each source, if a packet arrives on a link
that the local router believes to be on the shortest path back toward
the packet's source, then the router forwards the packet on all
interfaces except the incoming interface.  If the packet does not
arrive on the interface that is on the shortest path back toward the
source, then the packet is discarded.  The interface over which the
router expects to receive multicast packets from a particular source is
referred to as the "parent" link.  The outbound links over which the
router forwards the multicast packet are called "child" links for this

This basic algorithm can be enhanced to reduce unnecessary packet
duplication.  If the local router making the forwarding decision can
determine whether a neighboring router on a child link is "downstream,"
then the packet is multicast toward the neighbor.  (A "downstream"
neighbor is a neighboring router which considers the local router to be
on the shortest path back toward a given source.)  Otherwise, the packet
is not forwarded on the potential child link since the local router
knows that the neighboring router will just discard the packet (since it
will arrive on a non-parent link for the source, relative to that
downstream router).


                                    |   ^
                                    |   :    shortest path back to the
                                    |   :     source for THIS router
                                    |   :
                               "parent link"
                           |               |
                --"child -|!1|           |!3| - "child --
                    link"  |    ROUTER     |      link"

Figure 7: Reverse Path Broadcasting - Forwarding Algorithm

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The information to make this "downstream" decision is relatively easy to
derive from a link-state routing protocol since each router maintains a
topological database for the entire routing domain.  If a distance-
vector routing protocol is employed, a neighbor can either advertise its
previous hop for the source as part of its routing update messages or
"poison reverse" the route toward a source if it is not on the
distribution tree for that source.  Either of these techniques allows an
upstream router to determine if a downstream neighboring router is on an
active branch of the delivery tree for a certain source.

Please refer to Figure 8 for a discussion describing the basic operation
of the enhanced RPB algorithm.


                 Source Station------>O
                                    A #
                                    + | +
                                   +  O  +
                                  +       +
                                 1         2
                                +           +
                               +             +
                              +               +
                          B  +                 +  C
                          O-#- - - - -3- - - - -#-O
                           +|+                 -|+
                          + | +               - | +
                         +  O  +             -  O  +
                        +       +           -       +
                       +         +         -         +
                      4           5       6           7
                     +             +     -             +
                    +               + E -               +
                   +                 + -                 +
                D #- - - - -8- - - - -#- - - - -9- - - - -# F
                  |                   |                   |
                  O                   O                   O


O   Leaf
+ + Shortest-path
- - Branch
#   Router

Figure 8: Reverse Path Broadcasting - Example

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Note that the source station (S) is attached to a leaf subnetwork
directly connected to Router A.  For this example, we will look at the
RPB algorithm from Router B's perspective. Router B receives the
multicast packet from Router A on link 1.  Since Router B considers link
1 to be the parent link for the (source, group) pair, it forwards the
packet on link 4, link 5, and the local leaf subnetworks if they contain
group members.  Router B does not forward the packet on link 3 because
it knows from routing protocol exchanges that Router C considers link 2
as its parent link for the source.  Router B knows that if it were to
forward the packet on link 3, it would be discarded by Router C since
the packet would not be arriving on Router C's parent link for this
source. RPB: Benefits and Limitations

The key benefit to reverse path broadcasting is that it is reasonably
efficient and easy to implement.  It does not require that the router
know about the entire spanning tree, nor does it require a special
mechanism to stop the forwarding process (as flooding does).  In
addition, it guarantees efficient delivery since multicast packets
always follow the "shortest" path from the source station to the
destination group.  Finally, the packets are distributed over multiple
links, resulting in better network utilization since a different tree is
computed for each source.

One of the major limitations of the RPB algorithm is that it does not
take into account multicast group membership when building the delivery
tree for a source.  As a result, datagrams may be unnecessarily
forwarded onto subnetworks that have no members in a destination group.

6.2.2 Truncated Reverse Path Broadcasting (TRPB)

Truncated Reverse Path Broadcasting (TRPB) was developed to overcome the
limitations of Reverse Path Broadcasting.  With information provided by
IGMP, multicast routers determine the group memberships on each leaf
subnetwork and avoid forwarding datagrams onto a leaf subnetwork if it
does not contain at least one member of a given destination group.  Thus,
the delivery tree is "truncated" by the router if a leaf subnetwork has
no group members.

Figure 9 illustrates the operation of TRPB algorithm.  In this example
the router receives a multicast packet on its parent link for the
Source.  The router forwards the datagram on interface 1 since that
interface has at least one member of G1.  The router does not forward
the datagram to interface 3 since this interface has no members in the
destination group.  The datagram is forwarded on interface 4 if and only
if a downstream router considers this subnetwork to be part of its
"parent link" for the Source.

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                                |   :
                                |   :     (Source, G1)
                           "parent link"
              "child link"     ___
          G1            _______|2|_____
           \           |               |
         G3\\ _____   ___    ROUTER   ___      ______ / G2
            \| hub |--|1|             |3|-----|switch|/
            /|_____|  ^--     ___     --^     |______|\
            /        ^ |______|4|_____|  ^            \
          G1        ^       .^---         ^            G3
                   ^      .^   |           ^
                  ^     .^  "child link"    ^
                 Forward       |             Truncate

Figure 9: Truncated Reverse Path Broadcasting - (TRPB)

TRPB removes some limitations of RPB but it solves only part of the
problem.  It eliminates unnecessary traffic on leaf subnetworks but it
does not consider group memberships when building the branches of the
delivery tree.

6.2.3 Reverse Path Multicasting (RPM)

Reverse Path Multicasting (RPM) is an enhancement to Reverse Path
Broadcasting and Truncated Reverse Path Broadcasting.

RPM creates a delivery tree that spans only:

    o  Subnetworks with group members, and

    o  Routers and subnetworks along the shortest
       path to subnetworks with group members.

RPM allows the source-based "shortest-path" tree to be pruned so that
datagrams are only forwarded along branches that lead to active members
of the destination group. Operation

When a multicast router receives a packet for a (source, group) pair,
the first packet is forwarded following the TRPB algorithm across all

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routers in the internetwork.  Routers on the edge of the network (which
have only leaf subnetworks) are called leaf routers.  The TRPB algorithm
guarantees that each leaf router will receive at least the first
multicast packet.  If there is a group member on one of its leaf
subnetworks, a leaf router forwards the packet based on this group
membership information.


                      | :
                      | : (Source, G)
                      | v
                    ^ |         *
                    , |         *
                    ^ |         *  o
                    , |         * /
                    o-#-o       #***********
                    ^ |\      ^ |\         *
                    ^ | o     ^ | G        *
                    , |       , |          *
                    ^ |       ^ |          *
                    , |       , |          *
                      #         #          #
                     /|\       /|\        /|\
                    o o o     o o o      G o G

 #    Router
 o    Leaf without group member
 G    Leaf with group member
***   Active Branch
---   Pruned Branch
,>,   Prune Message (direction of flow -->

Figure 10: Reverse Path Multicasting  (RPM)

If none of the subnetworks connected to the leaf router contain group
members, the leaf router may transmit a "prune" message on its parent
link, informing the upstream router that it should not forward packets
for this particular (source, group) pair on the child interface on which
it received the prune message.  Prune messages are sent just one hop
back toward the source.

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An upstream router receiving a prune message is required to store the
prune information in memory.  If the upstream router has no recipients
on local leaf subnetworks and has received prune messages from each
downstream neighbor on each of the child interfaces for this (source,
group) pair, then the upstream router does not need to receive
additional packets for the (source, group) pair.  This implies that the
upstream router can also generate a prune message of its own, one hop
further back toward the source.  This cascade of prune messages results
in an active multicast delivery tree, consisting exclusively of "live"
branches (i.e., branches that lead to active receivers).

Since both the group membership and internetwork topology can change
dynamically, the pruned state of the multicast delivery tree must be
refreshed periodically.  At regular intervals, the prune information
expires from the memory of all routers and the next packet for the
(source, group) pair is forwarded toward all downstream routers.  This
allows "stale state" (prune state for groups that are no longer active)
to be reclaimed by the multicast routers. Limitations

Despite the improvements offered by the RPM algorithm, there are still
several scaling issues that need to be addressed when attempting to
develop an Internet-wide delivery service.  The first limitation is that
multicast packets must be periodically flooded across every router in
the internetwork, onto every leaf subnetwork.  This flooding is wasteful
of bandwidth (until the updated prune state is constructed).

This "flood and prune" paradigm is very powerful, but it wastes
bandwidth and does not scale well, especially if there are receivers at
the edge of the delivery tree which are connected via low-speed
technologies (e.g., ISDN or modem).  Also, note that every router
participating in the RPM algorithm must either have a forwarding table
entry for a (source, group) pair, or have prune state information for
that (source, group) pair.

It is clearly wasteful (especially as the number of active sources and
groups increase) to place such a burden on routers that are not on every
(or perhaps any) active delivery tree.  Shared tree techniques are an
attempt to address these scaling issues, which become quite acute when
most groups' senders and receivers are sparsely distributed across the

6.3 Shared Tree Techniques

The most recent additions to the set of multicast forwarding techniques
are based on a shared delivery tree.  Unlike shortest-path tree
algorithms which build a source-based tree for each source, or each
(source, group) pair, shared tree algorithms construct a single delivery
tree that is shared by all members of a group.  The shared tree approach
is quite similar to the spanning tree algorithm except it allows the

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definition of a different shared tree for each group.  Stations wishing
to receive traffic for a multicast group must explicitly join the shared
delivery tree.  Multicast traffic for each group is sent and received
over the same delivery tree, regardless of the source.

6.3.1 Operation

A shared tree may involve a single router, or set of routers, which
comprise(s) the "core" of a multicast delivery tree.  Figure 11
illustrates how a single multicast delivery tree is shared by all
sources and receivers for a multicast group.


               Source        Source        Source
                  |             |             |
                  |             |             |
                  v             v             v

                 [#] * * * * * [#] * * * * * [#]
                  ^             *             ^
                  |             *             |
             join |             *             | join
                  |            [#]            |
                 [x]                         [x]
                  :                           :
                member                      member
                 host                        host


[#]  Shared Tree "Core" Routers
* *  Shared Tree Backbone
[x]  Member-hosts' directly-attached routers

Figure 11: Shared Multicast Delivery Tree


Similar to other multicast forwarding algorithms, shared tree algorithms
do not require that the source of a multicast packet be a member of a
destination group in order to send to a group.

6.3.2 Benefits

In terms of scalability, shared tree techniques have several advantages

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over source-based trees.  Shared tree algorithms make efficient use of
router resources since they only require a router to maintain state
information for each group, not for each source, or for each (source,
group) pair. (Remember that source-based tree techniques required all
routers in an internetwork to either a) be on the delivery tree for a
given source or (source, group) pair, or b) to have prune state for
that source or (source, group) pair:  So the entire internetwork must
participate in the source-based tree protocol.)  This improves the
scalability of applications with many active senders since the number of
source stations is no longer a scaling issue.  Also, shared tree
algorithms conserve network bandwidth since they do not require that
multicast packets be periodically flooded across all multicast routers
in the internetwork onto every leaf subnetwork.  This can offer
significant bandwidth savings, especially across low-bandwidth WAN
links, and when receivers sparsely populate the domain of operation.
Finally, since receivers are required to explicitly join the shared
delivery tree, data only ever flows over those links that lead to active

6.3.3 Limitations

Despite these benefits, there are still several limitations to protocols
that are based on a shared tree algorithm.  Shared trees may result in
traffic concentration and bottlenecks near core routers since traffic
from all sources traverses the same set of links as it approaches the
core.  In addition, a single shared delivery tree may create suboptimal
routes (a shortest path between the source and the shared tree, a
suboptimal path across the shared tree, a shortest path between the
egress core router and the receiver's directly attached router)
resulting in increased delay which may be a critical issue for some
multimedia applications.  (Simulations indicate that latency over a
shared tree may be approximately 10% larger than source-based trees in
many cases, but by the same token, this may be negligible for many
applications.)  Finally, expanding-ring searches are not supported
inside shared-tree domains.


Certain multicast routing protocols are designed to work well in
environments that have plentiful bandwidth and where it is reasonable
to assume that receivers are rather densely distributed.  In such
scenarios, it is very reasonable to use periodic flooding, or other
bandwidth-intensive techniques that would not necessarily be very
scalable over a wide-area network.  In section 8, we will examine
different protocols that are specifically geared toward efficient WAN
operation, especially for groups that have widely dispersed (i.e.,
sparse) membership.

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These routing protocols include:

o  Distance Vector Multicast Routing Protocol (DVMRP),

o  Multicast Extensions to Open Shortest Path First (MOSPF),

o  Protocol Independent Multicast - Dense Mode (PIM-DM).

These protocols' underlying designs assume that the amount of protocol
overhead (in terms of the amount of state that must be maintained by
each router, the number of router CPU cycles required, and the amount of
bandwidth consumed by protocol operation) is appropriate since receivers
densely populate the area of operation.

7.1. Distance Vector Multicast Routing Protocol (DVMRP)

The Distance Vector Multicast Routing Protocol (DVMRP) is a distance-
vector routing protocol designed to support the forwarding of multicast
datagrams through an internetwork.  DVMRP constructs source-based
multicast delivery trees using the Reverse Path Multicasting (RPM)
algorithm.  Originally, the entire MBone ran only DVMRP.  Today, over
half of the MBone routers still run some version of DVMRP.

DVMRP was first defined in RFC-1075.  The original specification was
derived from the Routing Information Protocol (RIP) and employed the
Truncated Reverse Path Broadcasting (TRPB) technique. The major
difference between RIP and DVMRP is that RIP calculates the next-hop
toward a destination, while DVMRP computes the previous-hop back toward
a source.  Since mrouted 3.0, DVMRP has employed the Reverse Path
Multicasting (RPM) algorithm.  Thus, the latest implementations of DVMRP
are quite different from the original RFC specification in many regards.
There is an active effort within the IETF Inter-Domain Multicast Routing
(IDMR) working group to specify DVMRP version 3 in a standard form.

The current DVMRP v3 Internet-Draft is:

    <draft-ietf-idmr-dvmrp-v3-04.txt>, or

7.1.1 Physical and Tunnel Interfaces

The ports of a DVMRP router may be either a physical interface to a
directly-attached subnetwork or a tunnel interface to another multicast-
capable island.  All interfaces are configured with a metric specifying
cost for the given port, and a TTL threshold that limits the scope of a
multicast transmission.  In addition, each tunnel interface must be
explicitly configured with two additional parameters:  The IP address of
the local router's tunnel interface and the IP address of the remote
router's interface.

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   TTL                                Scope
    0                                Restricted to the same host
    1                                Restricted to the same subnetwork
   15                                Restricted to the same site
   63                                Restricted to the same region
  127                                Worldwide
  191                                Worldwide; limited bandwidth
  255                                Unrestricted in scope

Table 1:   TTL Scope Control Values

A multicast router will only forward a multicast datagram across an
interface if the TTL field in the IP header is greater than the TTL
threshold assigned to the interface.  Table 1 lists the conventional
TTL values that are used to restrict the scope of an IP multicast.  For
example, a multicast datagram with a TTL of less than 16 is restricted
to the same site and should not be forwarded across an interface to
other sites in the same region.

TTL-based scoping is not always sufficient for all applications.
Conflicts arise when trying to simultaneously enforce limits on
topology, geography, and bandwidth.  In particular, TTL-based scoping
cannot handle overlapping regions, which is a necessary characteristic
of administrative regions.  In light of these issues, "administrative"
scoping was created in 1994, to provide a way to do scoping based on
multicast address.  Certain addresses would be usable within a given
administrative scope (e.g., a corporate internetwork) but would not be
forwarded onto the global MBone.  This allows for privacy, and address
reuse within the class D address space.  The range from to has been reserved for administrative scoping.  While
administrative scoping has been in limited use since 1994 or so, it has
yet to be widely deployed.  The IETF MBoneD working group is working on
the deployment of administrative scoping.  For additional information,
please see <draft-ietf-mboned-admin-ip-space-01.txt> or its successor,
entitled "Administratively Scoped IP Multicast."

7.1.2 Basic Operation

DVMRP implements the Reverse Path Multicasting (RPM) algorithm.
According to RPM, the first datagram for any (source, group) pair is
forwarded across the entire internetwork (providing the packet's TTL and
router interface thresholds permit this).  Upon receiving this traffic,
leaf routers may transmit prune messages back toward the source if there
are no group members on their directly-attached leaf subnetworks.  The

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prune messages remove all branches that do not lead to group members
from the tree, leaving a source-based shortest path tree.

After a period of time, the prune state for each (source, group) pair
expires to reclaim stale prune state (from groups that are no longer in
use).  If those groups are actually still in use, a subsequent datagram
for the (source, group) pair will be flooded across all downstream
routers.  This flooding will result in a new set of prune messages,
serving to regenerate the source-based shortest-path tree for this
(source, group) pair.  In current implementations of RPM (notably
DVMRP), prune messages are not reliably transmitted, so the prune
lifetime must be kept short to compensate for lost prune messages.

DVMRP also implements a mechanism to quickly "graft" back a previously
pruned branch of a group's delivery tree.  If a router that had sent a
prune message for a (source, group) pair discovers new group members on
a leaf network, it sends a graft message to the previous-hop router for
this source.  When an upstream router receives a graft message, it
cancels out the previously-received prune message.  Graft messages
cascade (reliably) hop-by-hop back toward the source until they reach
the nearest "live" branch point on the delivery tree.  In this way,
previously-pruned branches are quickly restored to a given delivery

7.1.3 DVMRP Router Functions

In  Figure 13, Router C is downstream and may potentially receive
datagrams from the source subnetwork from Router A or Router B.  If
Router A's metric to the source subnetwork is less than Router B's
metric, then Router A is dominant over Router B for this source.

This means that Router A will forward any traffic from the source
subnetwork and Router B will discard traffic received from that source.
However, if Router A's metric is equal to Router B's metric, then the
router with the lower IP address on its downstream interface (child
link) becomes the Dominant Router for this source.  Note that on a
subnetwork with multiple routers forwarding to groups with multiple
sources, different routers may be dominant for each source.

7.1.4 DVMRP Routing Table

The DVMRP process periodically exchanges routing table updates with its
DVMRP neighbors.  These updates are logically independent of those
generated by any unicast Interior Gateway Protocol.

Since the DVMRP was developed to route multicast and not unicast
traffic, a router will probably run multiple routing processes in
practice:  One to support the forwarding of unicast traffic and another
to support the forwarding of multicast traffic. (This can be convenient:
A router can be configured to only route multicast IP, with no unicast

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              .-<-<-<-<-<-<-Source Subnetwork->->->->->->->->--.
              v                                                v
              |                                                |
          parent link                                      parent link
              |                                                |
        _____________                                    _____________
       | Router A    |                                  | Router B    |
       |             |                                  |             |
        -------------                                    -------------
              |                                                |
         child link                                       child link
              |                                                |
                                  parent link
                                | Router C    |
                                |             |
                                  child link

Figure 12. DVMRP Dominant Router in a Redundant Topology

IP routing.  This may be a useful capability in firewalled

Again, consider Figure 12:  There are two types of routers in this
figure:  dominant and subordinate; assume in this example that Router B
is dominant, Router A is subordinate, and Router C is part of the
downstream distribution tree.  In general, which routers are dominant
or subordinate may be different for each source!  A subordinate router
is one that is NOT on the shortest path tree back toward a source.  The
dominant router can tell this because the subordinate router will
'poison-reverse' the route for this source in its routing updates which
are sent on the common LAN (i.e., Router A sets the metric for this
source to 'infinity').  The dominant router keeps track of subordinate
routers on a per-source basis...it never needs or expects to receive a
prune message from a subordinate router.  Only routers that are truly on
the downstream distribution tree will ever need to send prunes to the
dominant router.  If a dominant router on a LAN has received either a
poison-reversed route for a source, or prunes for all groups emanating
from that source subnetwork, then it may itself send a prune upstream

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toward the source (assuming also that IGMP has told it that there are no
local receivers for any group from this source).

A sample routing table for a DVMRP router is shown in Figure 13.  Unlike


    Source      Subnet     From           Metric   Status   TTL
     Prefix      Mask       Gateway       3        Up     200       5        Up     150       2        Up     150       4        Up     200

Figure 13: DVMRP Routing Table

the table that would be created by a unicast routing protocol such as
the RIP, OSPF, or the BGP, the DVMRP routing table contains Source
Prefixes and From-Gateways instead of Destination Prefixes and Next-Hop

The routing table represents the shortest path (source-based) spanning
tree to every possible source prefix in the internetwork--the Reverse
Path Broadcasting (RPB) tree.  The DVMRP routing table does not
represent group membership or received prune messages.

The key elements in DVMRP routing table include the following items:

Source Prefix          A subnetwork which is a potential or actual
                       source of multicast datagrams.

Subnet Mask            The subnet mask associated with the Source
                       Prefix.  Note that the DVMRP provides the subnet
                       mask for each source subnetwork (in other words,
                       the DVMRP is classless).

From-Gateway           The previous-hop router leading back toward a
                       particular Source Prefix.

TTL                    The time-to-live is used for table management
                       and indicates the number of seconds before an
                       entry is removed from the routing table.  This
                       TTL has nothing at all to do with the TTL used
                       in TTL-based scoping.

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7.1.5 DVMRP Forwarding Table

Since the DVMRP routing table is not aware of group membership, the
DVMRP process builds a forwarding table based on a combination of the
information contained in the multicast routing table, known groups, and
received prune messages.  The forwarding table represents the local
router's understanding of the shortest path source-based delivery tree
for each (source, group) pair--the Reverse Path Multicasting (RPM) tree.


    Source      Multicast     TTL   InIntf    OutIntf(s)
     Prefix      Group     200    1 Pr      2p3p
           100    1         2p3
           250    1         2     150    2         2p3

Figure 14: DVMRP Forwarding Table

The forwarding table for a sample DVMRP router is shown in Figure 14.
The elements in this display include the following items:

Source Prefix           The subnetwork sending multicast datagrams
                        to the specified groups (one group per row).

Multicast Group         The Class D IP address to which multicast
                        datagrams are addressed.  Note that a given
                        Source Prefix may contain sources for several
                        Multicast Groups.

InIntf                  The parent interface for the (source, group)
                        pair.  A 'Pr' in this column indicates that a
                        prune message has been sent to the upstream
                        router (the From-Gateway for this Source Prefix
                        in the DVMRP routing table).

OutIntf(s)              The child interfaces over which multicast
                        datagrams for this (source, group) pair are
                        forwarded.  A 'p' in this column indicates
                        that the router has received a prune message(s)
                        from a (all) downstream router(s) on this port.

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7.2. Multicast Extensions to OSPF (MOSPF)

Version 2 of the Open Shortest Path First (OSPF) routing protocol is
defined in RFC-1583.  OSPF is an Interior Gateway Protocol (IGP) that
distributes unicast topology information among routers belonging to a
single OSPF "Autonomous System."  OSPF is based on link-state algorithms
which permit rapid route calculation with a minimum of routing protocol
traffic.  In addition to efficient route calculation, OSPF is an open
standard that supports hierarchical routing, load balancing, and the
import of external routing information.

The Multicast Extensions to OSPF (MOSPF) are defined in RFC-1584.  MOSPF
routers maintain a current image of the network topology through the
unicast OSPF link-state routing protocol.  The multicast extensions to
OSPF are built on top of OSPF Version 2 so that a multicast routing
capability can be incrementally introduced into an OSPF Version 2
routing domain.  Routers running MOSPF will interoperate with non-MOSPF
routers when forwarding unicast IP data traffic.  MOSPF does not support

7.2.1 Intra-Area Routing with MOSPF

Intra-Area Routing describes the basic routing algorithm employed by
MOSPF.  This elementary algorithm runs inside a single OSPF area and
supports multicast forwarding when a source and all destination group
members reside in the same OSPF area, or when the entire OSPF Autonomous
System is a single area (and the source is inside that area...).  The
following discussion assumes that the reader is familiar with OSPF. Local Group Database

Similar to all other multicast routing protocols, MOSPF routers use the
Internet Group Management Protocol (IGMP) to monitor multicast group
membership on directly-attached subnetworks.  MOSPF routers maintain a
"local group database" which lists directly-attached groups and
determines the local router's responsibility for delivering multicast
datagrams to these groups.

On any given subnetwork, the transmission of IGMP Host Membership
Queries is performed solely by the Designated Router (DR).  However,
the responsibility of listening to IGMP Host Membership Reports is
performed by not only the Designated Router (DR) but also the Backup
Designated Router (BDR).  Therefore, in a mixed LAN containing both
MOSPF and OSPF routers, an MOSPF router must be elected the DR for the
subnetwork.  This can be achieved by setting the OSPF RouterPriority to
zero in each non-MOSPF router to prevent them from becoming the (B)DR.

The DR is responsible for communicating group membership information to
all other routers in the OSPF area by flooding Group-Membership LSAs.
Similar to Router-LSAs and Network-LSAs, Group-Membership LSAs are only
flooded within a single area.

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The datagram's shortest path tree describes the path taken by a
multicast datagram as it travels through the area from the source
subnetwork to each of the group members' subnetworks.  The shortest
path tree for each (source, group) pair is built "on demand" when a
router receives the first multicast datagram for a particular (source,
group) pair.

When the initial datagram arrives, the source subnetwork is located in
the MOSPF link state database.  The MOSPF link state database is simply
the standard OSPF link state database with the addition of Group-
Membership LSAs.  Based on the Router- and Network-LSAs in the OSPF
link state database, a source-based shortest-path tree is constructed
using Dijkstra's algorithm.  After the tree is built, Group-Membership
LSAs are used to prune the tree such that the only remaining branches
lead to subnetworks containing members of this group.  The output of
these algorithms is a pruned source-based tree rooted at the datagram's


                   A  #
                     / \
                    /   \
                   1     2
                  /       \
               B #         # C
                / \         \
               /   \         \
              3     4         5
             /       \         \
          D #         # E       # F
                     / \         \
                    /   \         \
                   6     7         8
                  /       \         \
               G #         # H       # I


 #   Router

Figure 15. Shortest Path Tree for a (S, G) pair

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To forward multicast datagrams to downstream members of a group, each
router must determine its position in the datagram's shortest path tree.
Assume that Figure 15 illustrates the shortest path tree for a given
(source, group) pair.  Router E's upstream node is Router B and there
are two downstream interfaces:  one connecting to Subnetwork 6 and
another connecting to Subnetwork 7.

Note the following properties of the basic MOSPF routing algorithm:

    o  For a given multicast datagram, all routers within an OSPF
       area calculate the same source-based shortest path delivery
       tree.  Tie-breakers have been defined to guarantee that if
       several equal-cost paths exist, all routers agree on a single
       path through the area.  Unlike unicast OSPF, MOSPF does not
       support the concept of equal-cost multipath routing.

    o  Synchronized link state databases containing Group-Membership
       LSAs allow an MOSPF router to build a source-based shortest-
       path tree in memory, working forward from the source to the
       group member(s).  Unlike the DVMRP, this means that the first
       datagram of a new transmission does not have to be flooded to
       all routers in an area.

    o  The "on demand" construction of the source-based delivery tree
       has the benefit of spreading calculations over time, resulting
       in a lesser impact for participating routers.  Of course, this
       may strain the CPU(s) in a router if many new (source, group)
       pairs appear at about the same time, or if there are a lot of
       events which force the MOSPF process to flush and rebuild its
       forwarding cache.  In a stable topology with long-lived
       multicast sessions, these effects should be minimal. Forwarding Cache

Each MOSPF router makes its forwarding decision based on the contents of
its forwarding cache.  Contrary to DVMRP, MOSPF forwarding is not RPF-
based.  The forwarding cache is built from the source-based shortest-
path tree for each (source, group) pair, and the router's local group
database.  After the router discovers its position in the shortest path
tree, a forwarding cache entry is created containing the (source, group)
pair, its expected upstream interface, and the necessary downstream
interface(s).  The forwarding cache entry is now used to quickly
forward all subsequent datagrams from this source to this group.  If
a new source begins sending to a new group, MOSPF must first calculate
the distribution tree so that it may create a cache entry that can be
used to forward the packet.

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Figure 16 displays the forwarding cache for an example MOSPF router.
The elements in the display include the following items:

Dest. Group            A known destination group address to which
                       datagrams are currently being forwarded, or to
                       which traffic was sent "recently" (i.e., since
                       the last topology or group membership or other
                       event which (re-)initialized MOSPF's forwarding

Source                 The datagram's source host address.  Each (Dest.
                       Group, Source) pair uniquely identifies a
                       separate forwarding cache entry.


    Dest. Group     Source       Upstream     Downstream   TTL      11          12   13      5      11          12   13      2      11          12   13      3      12          11           7

Figure 16: MOSPF Forwarding Cache

Upstream               Datagrams matching this row's Dest. Group and
                       Source must be received on this interface.

Downstream             If a datagram matching this row's Dest. Group
                       and Source is received on the correct Upstream
                       interface, then it is forwarded across the listed
                       Downstream interfaces.

TTL                    The minimum number of hops a datagram must cross
                       to reach any of the Dest. Group's members.  An
                       MOSPF router may discard a datagram if it can see
                       that the datagram has insufficient TTL to reach
                       even the closest group member.

The information in the forwarding cache is not aged or periodically
refreshed:  It is maintained as long as there are system resources
available (e.g., memory) or until the next topology change.  The
contents of the forwarding cache will change when:

    o  The topology of the OSPF internetwork changes, forcing all of
       the shortest path trees to be recalculated.  (Once the cache
       has been flushed, entries are not rebuilt.  If another packet
       for one of the previous (Dest. Group, Source) pairs is

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       received, then a "new" cache entry for that pair will be
       created then.)

    o  There is a change in the Group-Membership LSAs indicating that
       the distribution of individual group members has changed.

7.2.2 Mixing MOSPF and OSPF Routers

MOSPF routers can be combined with non-multicast OSPF routers. This
permits the gradual deployment of MOSPF and allows experimentation with
multicast routing on a limited scale.

It is important to note that an MOSPF router is required to eliminate
all non-multicast OSPF routers when it builds its source-based shortest-
path delivery tree.  (An MOSPF router can determine the multicast
capability of any other router based on the setting of the multicast-
capable bit (MC-bit) in the Options field of each router's link state
advertisements.)  The omission of non-multicast routers may create a
number of potential problems when forwarding multicast traffic:

    o  The Designated Router for a multi-access network must be an
       MOSPF router.  If a non-multicast OSPF router is elected the
       DR, the subnetwork will not be selected to forward multicast
       datagrams since a non-multicast DR cannot generate Group-
       Membership LSAs for its subnetwork (because it is not running
       IGMP, so it won't hear IGMP Host Membership Reports).

    o  Even though there may be unicast connectivity to a destination,
       there may not be multicast connectivity.  For example, the only
       path between two points could require traversal of a non-
       multicast-capable OSPF router.

    o  The forwarding of multicast and unicast datagrams between two
       points may follow different paths, making some routing problems
       a bit more challenging to solve.

7.2.3 Inter-Area Routing with MOSPF

Inter-area routing involves the case where a datagram's source and some
of its destination group members reside in different OSPF areas.  It
should be noted that the forwarding of multicast datagrams continues to
be determined by the contents of the forwarding cache which is still
built from the local group database and the datagram source-based trees.
The major differences are related to the way that group membership
information is propagated and the way that the inter-area source-based
tree is constructed.

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In MOSPF, a subset of an area's Area Border Routers (ABRs) function as
"inter-area multicast forwarders."  An inter-area multicast forwarder is
responsible for the forwarding of group membership information and
multicast datagrams between areas.  Configuration parameters determine
whether or not a particular ABR also functions as an inter-area
multicast forwarder.

Inter-area multicast forwarders summarize their attached areas' group
membership information to the backbone by originating new Group-
Membership LSAs into the backbone area.  Note that the summarization of
group membership in MOSPF is asymmetric.  This means that while group
membership information from non-backbone areas is flooded into the
backbone, but group membership from the backbone (or from any other
non-backbone areas) is not flooded into any non-backbone area(s).

To permit the forwarding of multicast traffic between areas, MOSPF
introduces the concept of a "wild-card multicast receiver."  A wild-card
multicast receiver is a router that receives all multicast traffic
generated in an area.  In non-backbone areas, all inter-area multicast
forwarders operate as wild-card multicast receivers.  This guarantees
that all multicast traffic originating in any non-backbone area is
delivered to its inter-area multicast forwarder, and then if necessary
into the backbone area.  Since the backbone knows group membership for
all areas, the datagram can be forwarded to the appropriate location(s)
in the OSPF autonomous system, if only it is forwarded into the backbone
by the source area's multicast ABR. Inter-Area Datagram's Shortest-Path Tree

In the case of inter-area multicast routing, it is usually impossible to
build a complete shortest-path delivery tree.  Incomplete trees are a
fact of life because each OSPF area's complete topological and group
membership information is not distributed between OSPF areas.
Topological estimates are made through the use of wild-card receivers
and OSPF Summary-Links LSAs.

If the source of a multicast datagram resides in the same area as the
router performing the calculation, the pruning process must be careful
to ensure that branches leading to other areas are not removed from the
tree.  Only those branches having no group members nor wild-card
multicast receivers are pruned.  Branches containing wild-card multicast
receivers must be retained since the local routers do not know whether
there are any group members residing in other areas.

If the source of a multicast datagram resides in a different area than
the router performing the calculation, the details describing the local
topology surrounding the source station are not known.  However, this
information can be estimated using information provided by Summary-Links
LSAs for the source subnetwork.  In this case, the base of the tree

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begins with branches directly connecting the source subnetwork to each
of the local area's inter-area multicast forwarders.  Datagrams sourced
from outside the local area will enter the area via one of its inter-
area multicast forwarders, so they all must be part of the candidate
distribution tree.

Since each inter-area multicast forwarder is also an ABR, it must
maintain a separate link state database for each attached area.  Thus
each inter-area multicast forwarder is required to calculate a separate
forwarding tree for each of its attached areas.

7.2.4 Inter-Autonomous System Multicasting with MOSPF

Inter-Autonomous System multicasting involves the situation where a
datagram's source or some of its destination group members are in
different OSPF Autonomous Systems.  In OSPF terminology, "inter-AS"
communication also refers to connectivity between an OSPF domain and
another routing domain which could be within the same Autonomous System
from the perspective of an Exterior Gateway Protocol.

To facilitate inter-AS multicast routing, selected Autonomous System
Boundary Routers (ASBRs) are configured as "inter-AS multicast
forwarders."  MOSPF makes the assumption that each inter-AS multicast
forwarder executes an inter-AS multicast routing protocol which forwards
multicast datagrams in a reverse path forwarding (RPF) manner.  Since
the publication of the MOSPF RFC, a term has been defined for such a
router:  Multicast Border Router.  See section 9 for an overview of the
MBR concepts.  Each inter-AS multicast forwarder is a wildcard multicast
receiver in each of its attached areas.  This guarantees that each
inter-AS multicast forwarder remains on all pruned shortest-path trees
and receives all multicast datagrams.

The details of inter-AS forwarding are very similar to inter-area
forwarding.  On the "inside" of the OSPF domain, the multicast ASBR
must conform to all the requirements of intra-area and inter-area
forwarding.  Within the OSPF domain, group members are reached by the
usual forward path computations, and paths to external sources are
approximated by a reverse-path source-based tree, with the multicast
ASBR standing in for the actual source.  When the source is within the
OSPF AS, and there are external group members, it falls to the inter-
AS multicast forwarders, in their role as wildcard receivers, to make
sure that the data gets out of the OSPF domain and sent off in the
correct direction.

7.3 Protocol-Independent Multicast (PIM)

The Protocol Independent Multicast (PIM) routing protocols have been
developed by the Inter-Domain Multicast Routing (IDMR) working group of
the IETF.  The objective of the IDMR working group is to develop one--or
possibly more than one--standards-track multicast routing protocol(s)

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that can provide scaleable multicast routing across the Internet.

PIM is actually two protocols:  PIM - Dense Mode (PIM-DM) and PIM -
Sparse Mode (PIM-SM).  In the remainder of this introduction, any
references to "PIM" apply equally well to either of the two protocols...
there is no intention to imply that there is only one PIM protocol.
While PIM-DM and PIM-SM share part of their names, and they do have
related control messages, they are actually two completely independent

PIM receives its name because it is not dependent on the mechanisms
provided by any particular unicast routing protocol.  However, any
implementation supporting PIM requires the presence of a unicast routing
protocol to provide routing table information and to adapt to topology

PIM makes a clear distinction between a multicast routing protocol that
is designed for dense environments and one that is designed for sparse
environments.  Dense-mode refers to a protocol that is designed to
operate in an environment where group members are relatively densely
packed and bandwidth is plentiful.  Sparse-mode refers to a protocol
that is optimized for environments where group members are distributed
across many regions of the Internet and bandwidth is not necessarily
widely available.  It is important to note that sparse-mode does not
imply that the group has a few members, just that they are widely
dispersed across the Internet.

The designers of PIM-SM argue that DVMRP and MOSPF were developed for
environments where group members are densely distributed, and bandwidth
is relatively plentiful.  They emphasize that when group members and
senders are sparsely distributed across a wide area, DVMRP and MOSPF
do not provide the most efficient multicast delivery service.  The
DVMRP periodically sends multicast packets over many links that do not
lead to group members, while MOSPF can send group membership
information over links that do not lead to senders or receivers.

7.3.1 PIM - Dense Mode (PIM-DM)

While the PIM architecture was driven by the need to provide scaleable
sparse-mode delivery trees, PIM also defines a new dense-mode protocol
instead of relying on existing dense-mode protocols such as DVMRP and
MOSPF.  It is envisioned that PIM-DM would be deployed in resource rich
environments, such as a campus LAN where group membership is relatively
dense and bandwidth is likely to be readily available.  PIM-DM's control
messages are similar to PIM-SM's by design.

[This space was intentionally left blank.]

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PIM - Dense Mode (PIM-DM) is similar to DVMRP in that it employs the
Reverse Path Multicasting (RPM) algorithm.  However, there are several
important differences between PIM-DM and DVMRP:

    o  To find routes back to sources, PIM-DM relies on the presence
       of an existing unicast routing table.  PIM-DM is independent of
       the mechanisms of any specific unicast routing protocol.  In
       contrast, DVMRP contains an integrated routing protocol that
       makes use of its own RIP-like exchanges to build its own unicast
       routing table (so a router may orient itself with respect to
       active source(s)).  MOSPF augments the information in the OSPF
       link state database, thus MOSPF must run in conjunction with

    o  Unlike the DVMRP which calculates a set of child interfaces for
       each (source, group) pair, PIM-DM simply forwards multicast
       traffic on all downstream interfaces until explicit prune
       messages are received.  PIM-DM is willing to accept packet
       duplication to eliminate routing protocol dependencies and
       to avoid the overhead inherent in determining the parent/child

For those cases where group members suddenly appear on a pruned branch
of the delivery tree, PIM-DM, like DVMRP, employs graft messages to
re-attach the previously pruned branch to the delivery tree.


The most recent additions to the set of multicast routing protocols are
called "sparse mode" protocols.  They are designed from a different
perspective than the "dense mode" protocols that we have already
examined.  Often, they are not data-driven, in the sense that forwarding
state is set up in advance, and they trade off using bandwidth liberally
(which is a valid thing to do in a campus LAN environment) for other
techniques that are much more suited to scaling over large WANs, where
bandwidth is scarce and expensive.

These emerging routing protocols include:

    o  Protocol Independent Multicast - Sparse Mode (PIM-SM), and

    o  Core-Based Trees (CBT).

While these routing protocols are designed to operate efficiently over a
wide area network where bandwidth is scarce and group members may be
quite sparsely distributed, this is not to imply that they are only
suitable for small groups.  Sparse doesn't mean small, rather it is
meant to convey that the groups are widely dispersed, and thus it is
wasteful to (for instance) flood their data periodically across the
entire internetwork.

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8.1  Protocol-Independent Multicast - Sparse Mode (PIM-SM)

As described previously, PIM also defines a "dense-mode" or source-based
tree variant.  Again, the two protocols are quite unique, and other than
control messages, they have very little in common.  Note that while PIM
integrates control message processing and data packet forwarding among
PIM-Sparse and -Dense Modes, PIM-SM and PIM-DM must run in separate
regions.  All groups in a region are either sparse-mode or dense-mode.

PIM-Sparse Mode (PIM-SM) has been developed to provide a multicast
routing protocol that provides efficient communication between members
of sparsely distributed groups--the type of groups that are likely to
be common in wide-area internetworks.  PIM's designers observed that
several hosts wishing to participate in a multicast conference do not
justify flooding the entire internetwork periodically with the group's
multicast traffic.

Noting today's existing MBone scaling problems, and extrapolating to a
future of ubiquitous multicast (overlaid with perhaps thousands of
small, widely dispersed groups), it is not hard to imagine that existing
multicast routing protocols will experience scaling problems.  To
eliminate these potential scaling issues, PIM-SM is designed to limit
multicast traffic so that only those routers interested in receiving
traffic for a particular group "see" it.

PIM-SM differs from existing dense-mode protocols in two key ways:

    o  Routers with adjacent or downstream members are required to
       explicitly join a sparse mode delivery tree by transmitting
       join messages.  If a router does not join the pre-defined
       delivery tree, it will not receive multicast traffic addressed
       to the group.

       In contrast, dense-mode protocols assume downstream group
       membership and forward multicast traffic on downstream links
       until explicit prune messages are received.  Thus, the default
       forwarding action of dense-mode routing protocols is to forward
       all traffic, while the default action of a sparse-mode protocol
       is to block traffic unless it has been explicitly requested.

    o  PIM-SM evolved from the Core-Based Trees (CBT) approach in that
       it employs the concept of a "core" (or rendezvous point (RP) in
       PIM-SM terminology) where receivers "meet" sources.

[This space was intentionally left blank.]

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                    S1                                      S2
                   ___|___                                 ___|___
                        |                                   |
                        |                                   |
                        #                                   #
                         \                                 /
                          \                               /
                      ________________// | \\_______________
                     /         _______/  |  \______         \
                     #         #         #         #         #
                  ___|___   ___|___   ___|___   ___|___   ___|___
                        |     |   |        |     |            |
                        R     R   R        R     R            R

   #   PIM Router
   R   Multicast Receiver

Figure 17: Rendezvous Point

When joining a group, each receiver uses IGMP to notify its directly-
attached router, which in turn joins the multicast delivery tree by
sending an explicit PIM-Join message hop-by-hop toward the group's
RP.  A source uses the RP to announce its presence, and act as a conduit
to members that have joined the group.  This model requires sparse-mode
routers to maintain a bit of state (the RP-set for the sparse-mode
region) prior to the arrival of data.  In contrast, because dense-mode
protocols are data-driven, they do not store any state for a group until
the arrival of its first data packet.

There is only one RP-set per sparse-mode domain, not per group.
Moreover, the creator of a group is not involved in RP selection.  Also,
there is no such concept as a "primary" RP.  Each group has precisely
one RP at any given time.  In the event of the failure of an RP, a new
RP-set is distributed which does not include the failed RP.

8.1.1 Directly Attached Host Joins a Group

When there is more than one PIM router connected to a multi-access LAN,
the router with the highest IP address is selected to function as the
Designated Router (DR) for the LAN.  The DR may or may not be
responsible for the transmission of IGMP Host Membership Query messages,
but does send Join/Prune messages toward the RP, and maintains the
status of the active RP for local senders to multicast groups.

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When the DR receives an IGMP Report message for a new group, the DR
determines if the group is RP-based or not by examining the group
address.  If the address indicates a SM group (by virtue of the group-
specific state that even inactive groups have stored in all PIM
routers), the DR performs a deterministic hash function over the
sparse-mode region's RP-set to uniquely determine the RP for the


                                      Source (S)
                                        / \
                                       /   \
                                      /     \
                                     #       #
                                    /         \
                   Designated      /           \
       Host      | Router         /             \  Rendezvous Point
            -----|- # - - - - - -#- - - - - - - -RP   for group G
      (receiver) |  ----Join-->  ----Join-->


   #   PIM Router                 RP  Rendezvous Point

Figure 18: Host Joins a Multicast Group

After performing the lookup, the DR creates a multicast forwarding entry
for the (*, group) pair and transmits a unicast PIM-Join message toward
the primary RP for this specific group.  The (*, group) notation
indicates an (any source, group) pair.  The intermediate routers forward
the unicast PIM-Join message, creating a forwarding entry for the
(*, group) pair only if such a forwarding entry does not yet exist.
Intermediate routers must create a forwarding entry so that they will be
able to forward future traffic downstream toward the DR which originated
the PIM-Join message.

8.1.2 Directly Attached Source Sends to a Group

When a source first transmits a multicast packet to a group, its DR
forwards the datagram to the primary RP for subsequent distribution
along the group's delivery tree.  The DR encapsulates the initial

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multicast packets in a PIM-SM-Register packet and unicasts them toward
the primary RP for the group.  The PIM-SM-Register packet informs the
RP of a new source which causes the active RP to transmit PIM-Join
messages back toward the source's DR.  The routers between the RP and
the source's DR use the received PIM-Join messages (from the RP) to
create forwarding state for the new (source, group) pair.  Now all
routers from the active RP for this sparse-mode group to the source's DR
will be able to forward future unencapsulated multicast packets from
this source subnetwork to the RP.  Until the (source, group) state has
been created in all the routers between the RP and source's DR, the DR
must continue to send the source's multicast IP packets to the RP as
unicast packets encapsulated within unicast PIM-Register packets.  The
DR may stop forwarding multicast packets encapsulated in this manner
once it has received a PIM-Register-Stop message from the active RP for
this group.  The RP may send PIM-Register-Stop messages if there are no
downstream receivers for a group, or if the RP has successfully joined
the (source, group) tree (which originates at the source's DR).


                                Source (S)
                                   # v
                                  /.\ ,
                                 /  ^\ v
                                /    .\ ,
                               #      ^# v
                              /        .\ ,
             Designated      /          ^\ v
 Host      | Router         /            .\ ,             |       Host
      -----|-#- - - - - - -#- - - - - - - -RP- - - # - - -|-----
(receiver) |  <~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~   ~ ~ ~ ~ ~>   | (receiver)


   #   PIM Router
   RP  Rendezvous Point
> , >  PIM-Register
< . <  PIM-Join
~ ~ ~  Resend to group members

Figure 19: Source sends to a Multicast Group

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8.1.3 Shared Tree (RP-Tree) or Shortest Path Tree (SPT)?

The RP-tree provides connectivity for group members but does not
optimize the delivery path through the internetwork.  PIM-SM allows
routers to either a) continue to receive multicast traffic over the
shared RP-tree, or b) subsequently create a source-based shortest-path
tree on behalf of their attached receiver(s).  Besides reducing the
delay between this router and the source (beneficial to its attached
receivers), the shared tree also reduces traffic concentration effects
on the RP-tree.

A PIM-SM router with local receivers has the option of switching to the
source's shortest-path tree (i.e., source-based tree) once it starts
receiving data packets from the source.  The change-over may be
triggered if the data rate from the source exceeds a predefined
threshold.  The local receiver's last-hop router does this by sending a
Join message toward the active source.  After the source-based SPT is
active, protocol mechanisms allow a Prune message for the same source
to be transmitted to the active RP, thus removing this router from the
shared RP-tree.  Alternatively, the DR may be configured to continue
using the shared RP-tree and never switch over to the source-based SPT,
or a router could perhaps use a different administrative metric to
decide if and when to switch to a source-based tree.


                                          Source (S)
                                           % #
                                          % / \*
                                         % /   \*
                                        % /     \*
                    Designated         % #       #*
                     Router           % /         \*
                                     % /           \*
           Host      |  <-% % % % % % /             \v
                -----|-#- - - - - - -#- - - - - - - -RP
          (receiver) | <* * * * * * * * * * * * * * *

  #   PIM Router
  RP  Rendezvous Point
 * *  RP-Tree (Shared)
 % %  Shortest-Path Tree (Source-based)

Figure 20: Shared RP-Tree and Shortest Path Tree (SPT)

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Besides a last-hop router being able to switch to a source-based tree,
there is also the capability of the RP for a group to transition to a
source's shortest-path tree.  Similar controls (bandwidth threshhold,
administrative weights, etc.) can be used at an RP to influence these

8.2 Core Based Trees (CBT)

Core Based Trees is another multicast architecture that is based on a
shared delivery tree.  It is specifically intended to address the
important issue of scalability when supporting multicast applications
across the public Internet.

Similar to PIM-SM, CBT is protocol-independent.  CBT employs the
information contained in the unicast routing table to build its shared
delivery tree.  It does not care how the unicast routing table is
derived, only that a unicast routing table is present.  This feature
allows CBT to be deployed without requiring the presence of any specific
unicast routing protocol.

Another similarity to PIM-SM is that CBT has adopted the core discovery
mechanism ("bootstrap" ) defined in the PIM-SM specification.  For
inter-domain discovery, efforts are underway to standardize (or at least
separately specify) a common RP/Core discovery mechanism.  The intent is
that any shared tree protocol could implement this common discovery
mechanism using its own protocol message types.

In a significant departure from PIM-SM, CBT has decided to maintain it's
scaling characteristics by not offering the option of shifting from a
Shared Tree (e.g., PIM-SM's RP-Tree) to a Shortest Path Tree (SPT) to
optimize delay.  The designers of CBT believe that this is a critical
decision since when multicasting becomes widely deployed, the need for
routers to maintain large amounts of state information will become the
overpowering scaling factor.

Finally, unlike PIM-SM's shared tree state, CBT state is bi-directional.
Data may therefore flow in either direction along a branch.  Thus, data
from a source which is directly attached to an existing tree branch need
not be encapsulated.

8.2.1 Joining a Group's Shared Tree

A host that wants to join a multicast group issues an IGMP host
membership report.  This message informs its local CBT-aware router(s)
that it wishes to receive traffic addressed to the multicast group.
Upon receipt of an IGMP host membership report for a new group, the
local CBT router issues a JOIN_REQUEST hop-by-hop toward the group's
core router.

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If the JOIN_REQUEST encounters a router that is already on the group's
shared tree before it reaches the core router, then that router issues a
JOIN_ACK hop-by-hop back toward the sending router.  If the JOIN_REQUEST
does not encounter an on-tree CBT router along its path towards the
core, then the core router is responsible for responding with a
JOIN_ACK.  In either case, each intermediate router that forwards the
JOIN_REQUEST towards the core is required to create a transient "join
state."  This transient "join state" includes the multicast group, and
the JOIN_REQUEST's incoming and outgoing interfaces.  This information
allows an intermediate router to forward returning JOIN_ACKs along the
exact reverse path to the CBT router which initiated the JOIN_REQUEST.

As the JOIN_ACK travels towards the CBT router that issued the
JOIN_REQUEST, each intermediate router creates new "active state" for
this group.  New branches are established by having the intermediate
routers remember which interface is upstream, and which interface(s)
is(are) downstream.  Once a new branch is created, each child router
monitors the status of its parent router with a keepalive mechanism,
the CBT "Echo" protocol.  A child router periodically unicasts a
CBT_ECHO_REQUEST to its parent router, which is then required to respond
with a unicast CBT_ECHO_REPLY message.


                                            #- - - -#- - - - -#
                                                    |          \
                                                    |           #
                                                    # - - - - #
      member  |                                     |
       host --|                                     |
              |     --Join-->  --Join-->  --Join--> |
              |- [DR] - - - [:] - - - -[:] - - - - [@]
              |     <--ACK--   <--ACK--   <--ACK--


  [DR]  CBT Designated Router
   [:]  CBT Router
   [@]  Target Core Router
    #   CBT Router that is already on the shared tree

Figure 21: CBT Tree Joining Process

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If, for any reason, the link between an on-tree router and its parent
should fail, or if the parent router is otherwise unreachable, the
on-tree router transmits a FLUSH_TREE message on its child interface(s)
which initiates the tearing down of all downstream branches for the
multicast group.  Each downstream router is then responsible for
re-attaching itself (provided it has a directly attached group member)
to the group's shared delivery tree.

The Designated Router (DR) is elected by CBT's "Hello" protocol and
functions as THE single upstream router for all groups using that link.
The DR is not necessarily the best next-hop router to every core for
every multicast group.  The implication is that it is possible for a
JOIN_REQUEST to be redirected by the DR across a link to the best
next-hop router providing access a given group's core.  Note that data
traffic is never duplicated across a link, only JOIN_REQUESTs, and the
volume of this JOIN_REQUEST traffic should be negligible.

8.2.2 Data Packet Forwarding

When a JOIN_ACK is received by an intermediate router, it either adds
the interface over which the JOIN_ACK was received to an existing
forwarding cache entry, or creates a new entry if one does not already
exist for the multicast group.  When a CBT router receives a data packet
addressed to the multicast group, it simply forwards the packet over all
outgoing interfaces as specified by the forwarding cache entry for the

8.2.3 Non-Member Sending

Similar to other multicast routing protocols, CBT does not require that
the source of a multicast packet be a member of the multicast group.
However, for a multicast data packet to reach the active core for the
group, at least one CBT-capable router must be present on the non-member
source station's subnetwork.  The local CBT-capable router employs
IP-in-IP encapsulation and unicasts the data packet to the active core
for delivery to the rest of the multicast group.

8.2.4 CBT Multicast Interoperability

Multicast interoperability is currently being defined.  Work is underway
in the IDMR working group to describe the attachment of stub-CBT and
stub-PIM domains to a DVMRP backbone.  Future work will focus on
developing methods of connecting non-DVMRP transit domains to a DVMRP

CBT interoperability will be achieved through the deployment of domain
border routers (BRs) which enable the forwarding of multicast traffic
between the CBT and DVMRP domains.  The BR implements DVMRP and CBT on
different interfaces and is responsible for forwarding data across the
domain boundary.

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         /---------------\        /---------------\
         |               |        |                |
         |               |        |                |
         |    DVMRP      |--[BR]--|  CBT Domain    |
         |   Backbone    |        |                |
         |               |        |                |
         \---------------/        \---------------/

Figure 22: Domain Border Routers (BRs)

The BR is also responsible for exporting selected routes out of the CBT
domain into the DVMRP domain.  While the CBT stub domain never needs to
import routes, the DVMRP backbone needs to import routes to any sources
of traffic which are inside the CBT domain.  The routes must be imported
so that DVMRP can perform its RPF check.


In late 1996, the IETF IDMR working group began discussing a formal
structure that would describe the way different multicast routing
protocols should interact inside a multicast border router (MBR).  The
work can be found in the following internet draft:  <draft-thaler-
interop-00.ps>, or its successor.  The draft covers explicit rules for
the major multicast routing protocols that existed at the end of 1996:
DVMRP, MOSPF, PIM-DM, PIM-SM, and CBT, but applies to any potential
multicast routing protocol as well.

The IDMR standards work will focus on this generic inter-protocol MBR
scheme, rather than having to write 25 documents, 20 detailing how each
of those 5 protocols must interwork with the 4 others, plus 5 detailing
how two disjoint regions running the same protocol must interwork.

9.1  Requirements for Multicast Border Routers

In order to ensure reliable multicast delivery across a network with an
arbitrary mixture of multicast routing protocols, some constraints are
imposed to limit the scope of the problem space.

Each multicast routing domain, or region, may be connected in a "tree
of regions" topology.  If more arbitrary inter-regional topologies are
desired, a hierarchical multicast routing protocol (such as, H-DVMRP)
must be employed, because it carries topology information about how the
regions are interconnected.  Until this information is available, we
must consider the case of a tree of regions with one centrally-placed

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"backbone" region.  Each pair of regions is interconnected one or more

A MBR is responsible for injecting a default route into its "child
regions," and also injecting subnetwork reachability information into
its "parent region," optionally using aggregation techniques to reduce
the volume of the information while preserving its meaning.  MBRs which
comply with <draft-thaler-interop-00.ps> have other characteristics and
duties, including:

o  The MBR consists at least two active routing components, each
   an instance of some multicast routing protocol.  No assumption is
   made about the type of routing protocol (e.g., broadcast-and-prune
   or explicit-join; distance-vector or link-state; etc.) any component
   runs, or the nature of a "component".  Multiple components running
   the same protocol are allowed.

o  An MBR forwards packets between two or more independent regions, with
   one or more active interfaces per region, but only one component per

o  Each interface for which multicast is enabled is "owned" by exactly
   one of the components at a time.

o  All components share a common forwarding cache of (S,G) entries,
   which are created when data packets are received, and can be
   deleted at any time.  The component owning an interface is the only
   component that may change forwarding cache entries pertaining to
   that interface.  Each forwarding cache entry has a single incoming
   interface (iif) and a list of outgoing interfaces (oiflist).

[This space was intentionally left blank.]

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10.1 Requests for Comments (RFCs)

    1075   "Distance Vector Multicast Routing Protocol," D. Waitzman,
            C. Partridge, and S. Deering, November 1988.

    1112   "Host Extensions for IP Multicasting," Steve Deering,
            August 1989.

    1583   "OSPF Version 2," John Moy, March 1994.

    1584   "Multicast Extensions to OSPF," John Moy, March 1994.

    1585   "MOSPF: Analysis and Experience," John Moy, March 1994.

    1700   "Assigned Numbers," J. Reynolds and J. Postel, October
            1994. (STD 2)

    1812   "Requirements for IP version 4 Routers," Fred Baker,
            Editor, June 1995

    2000   "Internet Official Protocol Standards," Jon Postel,
            Editor, February 1997.

10.2 Internet-Drafts

   "Core Based Trees (CBT) Multicast: Architectural Overview,"
    <draft-ietf-idmr-cbt-arch-04.txt>, A. J. Ballardie, March 1997.

   "Core Based Trees (CBT) Multicast: Protocol Specification," <draft-
    ietf-idmr-cbt-spec-07.txt>, A. J. Ballardie, March 1997.

   "Core Based Tree (CBT) Multicast Border Router Specification for
    Connecting a CBT Stub Region to a DVMRP Backbone," <draft-ietf-
    idmr-cbt-dvmrp-00.txt>, A. J. Ballardie, March 1997.

   "Distance Vector Multicast Routing Protocol," <draft-ietf-idmr-
    dvmrp-v3-04.ps>, T. Pusateri, February 19, 1997.

   "Internet Group Management Protocol, Version 2," <draft-ietf-
    idmr-igmp-v2-06.txt>, William Fenner, January 22, 1997.

   "Internet Group Management Protocol, Version 3," <draft-cain-
    igmp-00.txt>, Brad Cain, Ajit Thyagarajan, and Steve Deering,

   "Protocol Independent Multicast-Dense Mode (PIM-DM): Protocol
    Specification," <draft-ietf-idmr-pim-dm-spec-04.ps>, D. Estrin,
    D. Farinacci, A. Helmy, V. Jacobson, and L. Wei, September 12, 1996.

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   "Protocol Independent Multicast-Sparse Mode (PIM-SM): Motivation
    and Architecture," <draft-ietf-idmr-pim-arch-04.ps>, S. Deering,
    D. Estrin, D. Farinacci, V. Jacobson, C. Liu, and L. Wei,
    November 19, 1996.

   "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol
    Specification," <draft-ietf-idmr-pim-sm-spec-09.ps>, D.  Estrin,
    D. Farinacci, A. Helmy, D. Thaler; S. Deering, M. Handley,
    V. Jacobson, C. Liu, P. Sharma, and L. Wei, October 9, 1996.

   (Note:  Results of IESG review were announced on December 23, 1996:
    This internet-draft is to be published as an Experimental RFC.)

   "PIM Multicast Border Router (PMBR) specification for connecting
    PIM-SM domains to a DVMRP Backbone," <draft-ietf-mboned-pmbr-
    spec-00.txt>, D. Estrin, A. Helmy, D. Thaler, Febraury 3, 1997.

   "Administratively Scoped IP Multicast," <draft-ietf-mboned-admin-ip-
    space-01.txt>, D. Meyer, December 23, 1996.

   "Interoperability Rules for Multicast Routing Protocols," <draft-
    thaler-interop-00.txt>, D. Thaler, November 7, 1996.

    See the IDMR home pages for an archive of specifications:


10.3 Textbooks

    Comer, Douglas E. Internetworking with TCP/IP Volume 1 Principles,
    Protocols, and Architecture Second Edition, Prentice Hall, Inc.
    Englewood Cliffs, New Jersey, 1991

    Huitema, Christian. Routing in the Internet, Prentice Hall, Inc.
    Englewood Cliffs, New Jersey, 1995

    Stevens, W. Richard. TCP/IP Illustrated: Volume 1 The Protocols,
    Addison Wesley Publishing Company, Reading MA, 1994

    Wright, Gary and W. Richard Stevens. TCP/IP Illustrated: Volume 2
    The Implementation, Addison Wesley Publishing Company, Reading MA,

10.4 Other

    Deering, Steven E. "Multicast Routing in a Datagram
    Internetwork," Ph.D. Thesis, Stanford University, December 1991.

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INTERNET-DRAFT      Introduction to IP Multicast Routing      March 1997

    Ballardie, Anthony J. "A New Approach to Multicast Communication
    in a Datagram Internetwork," Ph.D. Thesis, University of London,
    May 1995.

   "Hierarchical Distance Vector Multicast Routing for the MBone,"
    Ajit Thyagarajan and Steve Deering, July 1995.


Security issues are not discussed in this memo.


This RFC would not have been possible without the encouragement of Mike
O'Dell and the support of Joel Halpern and David Meyer.  Also invaluable
was the feedback and comments of the IETF MBoneD and IDMR working groups.
Certain people spent considerable time commenting on and discussing this
paper with the authors, and deserve to be mentioned by name:  Tony
Ballardie, Steve Casner, Jon Crowcroft, Steve Deering, Bill Fenner, Hugh
Holbrook, Cyndi Jung, Shuching Shieh, Dave Thaler, and Nair Venugopal.
Our apologies to anyone we unintentionally neglected to list here.


    Tom Maufer
      3Com Corporation
      5400 Bayfront Plaza
      P.O. Box 58145
      Santa Clara, CA 95052-8145

      Phone:  +1 408 764-8814
      Email:  <maufer@3Com.com>

    Chuck Semeria
      3Com Corporation
      5400 Bayfront Plaza
      P.O. Box 58145
      Santa Clara, CA 95052-8145

      Phone:  +1 408 764-7201
      Email:  <semeria@3Com.com>

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